Differential Derecruitment Thresholds of the Long and Short Heads of the Biceps Brachii in Young and Old Men.
Bibliographic record
Abstract
Results from prior studies in this laboratory indicate that motor unit (MU) discharge rate differs between the long (LBB) and short (SBB) head of the biceps brachii, and changes when forearm position is altered between neutral, pronation and supination. To-date, no study has investigated the influence of forearm position and age on MU derecruitment. PURPOSE: To determine whether motor unit derecruitment differs between the LBB and SBB of young and old men during performance of isometric elbow flexion in three forearm positions. METHODS: Five young (22±3yrs) and five old (84±3yrs) men performed isometric contractions in one of three forearm positions (neutral, supination, and pronation). Each contraction began with a 7.5s ramped increase in force to 10% followed by a 20s period of constant force (10% maximum), ending with a 7.5s ramp-down contraction to rest. Intramuscular EMG was recorded from the LBB and SBB. The force relative to maximal voluntary contraction (MVC) at which cessation of continuous motor unit discharge occurred was termed the derecruitment threshold. RESULTS: Young men (5.4±2.5%MVC) demonstrated higher derecruitment thresholds compared with old men (3.9±2.5%MVC) (p<0.05). Despite a difference in MU derecruitment between young and old men, both groups responded similarly to a change in forearm position, and thus age was made a covariate for further analyses. Two factor ANCOVA revealed an interaction of head of biceps brachii and forearm position (p<0.05). In the SBB, the pronated position (5.1±2.8%MVC) had the highest derecruitment threshold followed by the neutral (4.4±1.8%MVC) and supinated (4.1±3.2%MVC) positions. In the LBB, the supinated position (6.1±2.3%MVC) had the highest derecruitment threshold followed by neutral (5.6±2.8%MVC) and pronated (3.1±1.9%MVC). CONCLUSION: Data confirm previous findings of lower derecruitment thresholds in old adults compared with young adults, but extend the existing literature to suggest that age-related differences in MU derecruitment in the biceps brachii are dependent on the muscle head and forearm position. Supported by NSERC.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".